Glass strengthened by ion exchange and method of preparing the same

ABSTRACT

In a process of strengthening a glass article by substituting potassium ions for sodium ions in the surface layer of the article at an elevated temperature below the strain point of the glass, whereby a compressive stress is developed in the layer, a glass having a viscosity and devitrification characteristics similar to those of conventional sheet glass is employed. The glass consists essentially of 60 to 75% SiO24 to 15% Al2O3, 8 to 15% Na2O, 4 to 10% K2O, 1 to 8% MgO and/or ZnO by weight.

United States Patent Hara et a1. Nov. 13, 1973 [54] GLASS STRENGTHENEDBY ION 3,357,876 12/1967 Rinehart 161/1 EXCHANGE A METHOD OF PREPARING3,433,61 1 3/1969 Saunders et al. 65/30 THE SAME 3,287,200 11/1966 Hesset al. 65/30 3,445,316 5/1969 Megles 161/1 [75] Inventors: MorlhisaHara; Yoshlro Suzuki, 3,410,673 11/1968 Marusak I I 65/30 both of Tokyo;Hironori Oht 3,301,649 3/1964 Marusak 65/30 Michlhlko Uemura, both of3,495,967 2/1970 Buckley et a1. 161/1 Kanagawa-ken, all of Japan3,558,415 1/1971 Reiser et a1 161/199 Assignee: Asahi Glass Co., Ltd.,Tokyo, Japan Filed: Apr. 17, 1972 Appl. No.: 244,851

Related U.S. Application Data Continuation of Ser. No. 52,406, July 6,1970, abandoned.

Foreign Application Priority Data July 10, 1969 Japan 44/54101 US. Cl161/164, 65/30, 161/1,

161/165,161/192,161/199 Int. Cl..... C03c 21/00, B32b 5/14, B32b 17/10[58] Field of Search 161/1,164,165, 161/166, 192, 199; 65/30 [56]References Cited UNITED STATES PATENTS 1356.477 12/1967 Chisholm et a165/30 Primary Examiner-Daniel J. Fritsch Attorney-Hans Berman et al.

[57] ABSTRACT In a process of strengthening a glass article bysubstituting potassium ions for sodium ions in the surface layer of thearticle at an elevated temperature below the strain point of the glass,whereby a compressive stress is developed in the layer, a glass having aviscosity and devitrification characteristics similar to those ofconventional sheet glass is employed. The glass consists essentially of60 to 75% 510;, 4 to 15% A1 0 8 to 15% Na O, 4 to 10% K O,1 to 8% MgOand/or Zn0 by weight.

5 Claims, 2 Drawing Figures TEMPERATURE [U TEMPERATURE (1% K01 x 10Patented Nov .13, 1973 VISCOSITY [log 1; poise] VISCOSITY [lo oise]TEMPERATURE Ecj 0'8 09 lb If! [2 1.5 1:415 TEMPERATURE 6+ o)X :0

I\AI I300 n'00' 800 I000 I200 I400 TEMPERATURE 01 |NVENTOR- dr/i/sq H414, 7 55/10: f0 Zu H/rvhar/ 6 a, M/c//Ko (/em ur- A GEA/ T 5 CLASSSTRENGTHENED BY ION AND METHOD OF PREPARING THE SAME This application isa continuation of the copending application Ser. No. 52,406, filed onJuly 6, 1970, and now abandoned.

This invention relates to the strengthening of a glass article by ionexchange and to the article so strengthened. More particularly, thisinvention relates to the strengthening of a glass article by exchangingsodium ions present in the surface layer of the glass article forpotassium ions and to the strengthened glass article.

It is known to strengthen a glass article containing sodium ions bycontact with a molten salt containing alkali metal ions which have adiameter greater than that of the sodium ions. The potassium ions aresubstituted for those of sodium, and a compressive stress is developedin the surface layer of the glass article [Journal of the AmericanCeramic Society, Vol. 45, No. 2 (February 1962), pages 5967].

However, when this method is applied to a conventional soda-lime glass,such as window glass, drawn glass, float glass, rolled glass or plateglass, the rate of ion exchange is extremely slow. A stressed layerhaving a thickness of 30 to 40 microns is developed only in about 15 to20 hours. if it is attempted to increase the thickness of the stressedlayer by either prolonging the treating time or by elevating thetreating temperature, the strength of the glass article is reduced byrelaxation of the compressive stress.

The object of the present invention is an improvement in the prior artmethod of strengthening a glass article by ion exchange.

Another object of this invention is the provision of a glass compositionsuitable for continuous sheet glass forming processes, such as theFroucault process, Pennvernon or Pittsburgh process, Colburn process, orfloat process, and capable of acquiring sufficient strength in arelatively short period of time by ion exchange treatment.

A glass to be employed in the method according to the present inventionis required to retain viscosity and devitrification characteristics of aconventional sheet glass, that is, the following ranges of viscosityreference points and liquidus temperature:

Strain point-450 550 C Working point980 1,l50 C Liquidustemperature-below 1,100 C Strain point is the temperature at which theviscosity of glass is 10' poise; working point is the temperature atwhich the viscosity of glass is 10" poise.

We have found that a glass satisfying these conditions and sufficientlystrengthened by a short ion exchange treatment consists essentially, byweight, of 60 75% SiO 4 15% A1 0 3% TiO 0 3% ZrO the SiO A1 0 TiO andZrO constituting 70 82% by weight of the glass composition; 4 K 0; 8 NaO; l 8% RO, wherein R0 represents at least one divalent oxide selectedfrom the group consisitng of 0 1n the appended drawing:

FIG. 1 is a diagram showing the logarithm of viscosity (poise) as afunction of the reciprocal value of the absolute temperature K) for aglass of the invention and a float glass of conventional composition.

FIG. 2 shows the logarithm of viscosity as a function of temperature inC for the same glasses at temperatures higher than those of FIG. 1.

The glass to be strengthened must be within the composition limitsindicated above for the following reasons:

An excessive or insufficient content of SiO will cause easydevitrification and make the forming of sheet glass difficult. 1f theSiO content is too high, the glass becomes hard, and the melting andforming of the glass is difficult. However, the SiO content should be ashigh as possible for a good ion exchange rate. Therefore, SiO should be60 to 75 percent by weight.

While an A1 0 content in excess of 4 percent will increase the ionexchange rate, more than 15 percent will tend to devitrify the glass,and will increase the viscosity thereof.

TiO makes the glass soft, but colored when present in an excessiveamount. A small amount of ZrO inhibits devitrification of the glass, butcrystals of an excess are precipitated in the glass. Consequently, theTiO and ZrO should not exceed 3 percent each.

The combined amount of SiO A1 0 TiO and ZrO should be to 82 percent byweight of the glass composition for proper devitrification tendency,viscosity characteristics, melting and forming properties, and ionexchange effect.

K 0 increases the ion exchange rate. However, an excess will increasethe viscosity of the glass. Therefore, the K 0 content should be 4 to 10percent by weight.

Na O is the source of sodium ions for the ion exchange reaction. lfitexceeds 15 percent by weight, the viscosity of the glass will bereduced, and its strain point will be lowered so as to relax thecompressive stress developed by the ion exchange. On the other hand,with less than 8 percent Na O, the melting and forming properties of theglass article are impaired by an increase in viscosity, and the glassarticle tends to become devitrified.

MgO, ZnO, CaO, PbO or SrO, when exceeding 1 percent by weight, eachimprove the melting property of the glass, and the viscosity curve ismade suitable for glass forming. When any one of the divalent ions is inexcess of 8 percent by weight, the liquidus temperature of the glassrises, and the viscosity curve becomes nearly parallel with the ordinatein a chart such as F163. 1 and 2. Therefore, forming becomes difficult,and the ion exchange rate is lowered. MgO and ZnO are superior to otherdivalent ions in increasing the ion exchange rate, and may amount to 1to 8 percent by weight. When CaO, PbO, Ba() and SrO exceed 2 percent byweight, the ion exchange rate will be lowered.

B 0 is helpful in controlling the viscosity curve of the glass withoutcausing an excessive rise of the liquidus temperature. If B 0 exceeds 4percent, the glass tends to become nonuniform and the strain point islowered.

The components discussed in the foregoing paragraphs should constituteat least 98 percent of the glass composition.

Li O improves the melting and forming properties of the glass, but if itexceeds 2 percent by weight, the glass tends to be dcvitrified. For thisreason, the U content should not exceed 1 percent.

F, A8 0; and Sb O which are impurities. should preferably not exceed lpercent each.

sheet or plate glass, but it can be used with equal effectiveness in thetreatment of other glass articles, cg, glass ware.

Details as to the working of this invention are given The glasscomposition is prepared by melting a mix- 5 hereunder by way ofExamples. ture of raw materials in the desired ro ortions accordpEXAMPLE 1 ing to the conventional method, refining or homogenizmg themolten g and forming it into a desired Samples of the glasses showninTable l below were P y! the formed glass article is Subjected preparedby melting silica sand, potassium carbonate, to ion exchang r men o ly fr being annealed. magnesium carbonate, borax, lithium carbonate, feld-Bccause of its temperature-viscosity characteristics and Spar, l i h dxid bm-i a id, etalite and the liquidus temperature, the glass of theinvention is capalike in the proportions required in a 500 ml platinumble of being formed into sheet glass in continuous opercrucible forabout hours at l,500 C. ation. The glass sheet can be subjected to anappropri- Thereafter, each melt was pressed into sheet form ate ionexchange treatment after being bent. The bendand the glass sheet was cutinto samples 10 X 10 X 0.2 ing may be required for wind shields or rearwindows cm.

" TABLE 1 (llztss (lluss composition llltlclllll by wt.) himccciiwcvrrAin .77 fiir V7". VY V-. .77 c a, h ll? sun Alst); no?'lll); L120 m o 0 Mgt) Ziiu mm 1 A.. A

The strain point, working point, and liquidus temperature were measuredfor each sample. The results are shown in Table 2 below.

TABLE 2 Viscosity reference point.

a mixture of an inert solid material, such as clay, and strain WorkingLiquidus te nppro c ,t. i a potassium salt to the surface of the glassarticle, and Glasssample pomt( o. omrr c. .iturt then heating. 480 1,130No dcvitrilicatlmi.

The treating temperature depends on the composi- 40 490 1,060Nu(lnvltrllir-zrtlor1. tion of the glass article. It should be below thestrain lfi fi point of the glass and above about 350 C, preferably -53 ilower than the strain point of the glass by about 30 to 520 1,120 Do.

The thickness of the ion exchange layer increases approximately inproportion to the square root of the treating time. It should be noted,however, that the compressive stress in the surface layer of the glassarticle is relaxed by an increase in the treating time. Therefore,extended treatment is not practical. The preferred period of time forthe treatment ranges from about 30 minutes to 2 hours. In this manner,the thickness of the stressed layer is made 30 to microns, and thecompressive stress is 40 to 60 kg/mm The compressive stress value andthe thickness of the stressed surface layer are directly related to thestrengthening effect.

The glass to be treated according to the present invention iscontinuously formed into a flat glass ribbon or a sheet in aconventional process. A single glass sheet strengthened by ion exchangetreatment according to this invention can be used in windows for automobiles, airplanes and buildings, or a single sheet can be combined withan unstrengthened glass sheet or with a glass sheet strenthened eitherby tempering or by ion exchange to form a laminated glass product.

The method of the present invention is not limited to The liquidustemperature was measured by keeping a rod of the tested glass for 15hours in an electric furnace in which a temperature gradient wasprovided from 900 to 1,050" C, and by reading the maximumdevitrification temperature. in Table 2 N0 devitrifcation means that nodevitrification was observed at 900 to 1,050 C. The absence ofdevitrification observable by a microscope implies that either the glassarticle has a liquidus temperature lower than 900 C or the glass articlehas an extremely slow devitrification rate.

ln carrying out the ion exchange treatment, each sample sheet wasimmersed in a bath of molten potassium nitrate (KNO maintained at either430 C or 450 C for 1 hour. Thereafter, the sample sheet was removed fromthe bath and washed in water. Table 3 below shows the ion exchangetreating time, temperature and compressive stress values established onthe surface of the strengthened glass sheet, and also the thickness ofthe stressed layer.

The viscosity curve for the glass No. 5 ofTable l is shown in FIGS. Eand 2.

TABLE 3 Thickness Treating conditions of stressed Glass Compressivesurface sample Salt Temp. Period ress layer 0. bath 0.) (hr.) (kg/mm?)(microns) For comparative purposes the viscosity curve of a commercialfloat glass also is shown in FIGS. 1 and 2. The composition of the floatglass is as follows:

by weight SiO 72.2 Al,O 1.8 CaO 7.3 MgO 3.8 Na O 13.7 K 0.7 SO; 0.3 Fe o0.1 T 0.1

The liquidus temperature of this glass was 970 C.

From FIGS. 1 and 2, it will be realized that theglass treated by thepresent invention has a temperatureviscosityciiaiaefirfitiafiiiisximmilig that or'cofiiir tional sheet glass orplate glass.

@KAMRLEl,

EXMLP L 3 From a glass having the composition shown in Table ....N9LLahe Q X 3. 22592 em was.f9r med-.A o t-.

ventional float glass sheet X 30 X 0.3 cm was also prepared. Thereafter,the two glass sheets were bent. The curved glass of the invention wasstrengthened by ion exchange as i r lxam ple l, and laminated to saidfloat glass sheet by a polyvinyl butyral film having a thickness of 0.7mm and inserted between the two glass sheets as a bonding agent.

The mechanical strength of the laminate was measured in the followingmanner.

A rubber ring having a thickness of 2 mm wasset on a steel plate 5mg;F615 (T200 mm diameter, and the laminate was placed on the ring with thestrengthened glass sheet on the underside. A steel rod having a diam-.eter of 50 mm and a semispherical end was pushed downwardly to thecenter of the laminate until the sheet was destroyed. The strain in theglass sheet at the time of destruction was measured by a strain gauge.If the strain is e, Young's modulus E, and Poisson's ratio 'y, thebreaking stress 8 can be calculated by the formule...

It was found to be kglmm This compares with the breaking stress of alaminated glass sheet consisting of two unstrengthened glass sheetshaving thicknesses of 2 mm and 3 mm, which was found to be 10 kg/mm.

What is claimed is:

l. A glass article containing sodium ions and potassium ions, having asurface layer under compressive stress, the concentration of potassiumions in the surface layer being greater than in the interior of theglass article, the interior of said glass article consistingessentially, by weight, of 60 to SiO;; 4 to 15% A1 0 0 to 3% TiO 0 to 3%ZrO said SiO A1 0 TiO and ZrO constituting 70 to 82% by weight of theglass composition in said interior; 4 to 10% K 0; 8 to 15% Na O; 1 to 8%RO, wherein R0 represents at least one divalent oxide selected from thegroup consisting of 0 to 8% MgO, 0 to 8% Zn() and O to 2% CaO, PbO, BaO,SrO, and mixtures thereof; and O to 4 B 0 said SiO A1- 0 TiO ZrO K 0, NaO, R0 and i3 0 constituting at least 98% of said glass composition, theremainder of said composition essentially consisting of members of thegroup consisting of Li O, F, AS203, and Sb O each member of said groupnot exceeding 1 percent.

2. A glass article as claimed in claim 1, wherein the thickness of thelayer under compressive stress is at least about 30 microns, and saidcompressive stress is at least about 40 kglmm 3. A method ofstrengthening a glass article, which comprises replacing sodium ionspresent in the surface layer of the glass article by potassium ions atan elevated temperature lower than the strain point of the glass of saidarticle until a compressive stress is developed in said layer, whereinthe improvement comprises providing a glass of said article whichconsists essentially, by weight of 60 to 75% SiO 4 to 15% A1 0 0 to 3%TiO 0 to 3% ZrO said SiO A1 0 TiO and ZrO constituting 70 to 82% byweight of the glass composition; 4 10% K O', 8 to 15% Na O; 1 to 8% R0,wherein said R0 represents at least one divalent oxide selected from thegroup consisting of 0 to 8% MgO, 0 to 8% ZnO and 0 to 2% CaO, PbO, BaO,SrO, and mixtures thereof; and 0 to 4% H 0 said SiO A1 0 TiO ZrO K 0, NaO, R0 and E 0 constituting at least 98% of the glass composition, theremainder essentially consisting of members of the group consisting of U0 F, AS203, and Sb 0 each member of said group not exceeding onepercent.

4. A method as claimed in claim 3, said sodium ions being repalced bycontact of said glass article with molten potassium nitrate maintainedat a temperature lower than the strain point of said glass by about 30to 50 C.

5. A method as claimed in claim 4, contact of said glass article withsaid molten potassium nitrate being maintained for 30 minutes to 2 hoursuntil a compressive stress of at least about 40 kg/mm is developed in asurface layer of said article at least about 30 microns thick.

2. A glass article as claimed in claim 1, wherein the thickness of thelayer under compressive stress is at least about 30 microns, and saidcompressive stress is at least about 40 kg/mm2.
 3. A method ofstrengthening a glass article, which comprises replacing sodium ionspresent in the surface layer of the glass article by potassium ions atan elevated temperature lower than the strain point of the glass of saidarticle until a compressive stress is developed in said layer, whereinthe improvement comprises providing a glass of said article whichconsists essentially, by weight of 60 to 75% SiO2; 4 to 15% Al2O3; 0 to3% TiO2; 0 to 3% ZrO2; said SiO2, Al2O3, TiO2 and ZrO2 constituting 70to 82% by weight of the glass composition; 4 10% K2O; 8 to 15% Na2O; 1to 8% RO, wherein said RO represents at least one divalent oxideselected from the group consisting of 0 to 8% MgO, 0 to 8% ZnO and 0 to2% CaO, PbO, BaO, SrO, and mixtures thereof; and 0 to 4% B2O3; saidSiO2, Al2O3, TiO2, ZrO2, K2O, Na2O, RO and B2O3 constituting at least98% of the glass composition, the remainder essentially consisting ofmembers of the group consisting of Li2O, F, As2O3, and Sb2O3, eachmember of said group not exceeding one percent.
 4. A method as claimedin claim 3, said sodium ions being repalced by contact of said glassarticle with molten potassium nitrate maintained at a temperature lowerthan the strain point of said glass by about 30* to 50* C.
 5. A methodas claimed in claim 4, contact of said glass article with said moltenpotassium nitrate being maintained for 30 minutes to 2 hours until acompressive stress of at least about 40 kg/mm2 is developed in a surfacelayer of said article at least about 30 microns thick.